1. Field of the Invention
The present invention relates to a clock apparatus, and more particularly, to a clock apparatus having high accuracy and for use with a computer system or the like.
2. Description of the Related Art
Recently, computer systems have been used to realize non-stop operations over periods of several years, and thus a clock apparatus having high accuracy and a battery backup function has been required. Namely, the clock apparatus must count the time, even though a general power supply unit (which is driven by using a commercial power supply) is stopped, by using a battery (backup unit) instead of the general power supply.
Conventionally, the clock apparatus is, for example, constituted by a crystal clock having a clock integrated circuit and a quartz resonator or by an atomic clock having higher accuracy than the crystal clock.
First, the crystal clock counts the time by dividing an oscillation frequency of the quartz resonator and counting the divided signal, and the crystal clock has an accuracy about 10 ppm/month (corresponding to .+-.5 ppm/month, about 30 (25.9) seconds/month). Note that the crystal clock consumes little power, and thus a backup operation using a battery can be realized. However, the accuracy of the crystal clock is not sufficient for use with a continuously operating computer system or the like, since the accuracy of the crystal clock depends on various factors, e.g., the precision of the quartz resonator, the oscillation frequency of the quartz resonator which changes in accordance with a temperature, and the power supply voltage for driving the crystal clock.
On the other hand, the atomic clock counts the time by measuring a frequency radiated from an atom (for example, Cs: Cesium), and the atomic clock has an accuracy better than 0.5 ppm/month (corresponding to .+-.0. 25 ppm/month, about 1 (1.29) second/month). Note that the atomic clock has higher accuracy (sufficient for use with the continuously operating computer system) than the crystal clock, but consumes a large amount of power, so that a backup operation using a battery cannot be realized. Namely, in the case that the atomic clock is applied to a continuously operating computer system (for example, non-stop host computer, or non-stop server), when a general power supply stops providing general power (general power voltage) to the clock apparatus including the atomic clock, the atomic clock cannot in practice be driven by a battery.
Therefore, in the prior art, non-stop host computer or non-stop server including a clock apparatus having high accuracy, corresponding to that of an atomic clock, can not be provided.